AS THE Sun approaches the peak of its 11-year cycle of solar activity, the
Earth鈥檚 atmosphere will be buffeted almost daily by jets of solar material,
which can damage satellites, endanger astronauts and disrupt power grids. There
are a number of different theories to explain these violent eruptions, but last
week in Lake Tahoe, Nevada, the American Astronomical Society鈥檚 solar physics
division meeting heard how one of the most detailed observations to date of a
solar flare supports the so-called 鈥減ressure cooker鈥� model, in which energy and
gas spew from the Sun like overheated water.
In the pressure cooker model, a magnetic 鈥渓id鈥� traps bundles of magnetic
energy emerging from the visible solar surface and constrains them low in the
hot ionised gas of the corona鈥攖he outermost region of the Sun鈥檚
atmosphere. The magnetic energy then builds up until it bursts through a hole in
the lid, producing an eruptive flare or a larger coronal mass ejection.
According to Guillaume Aulanier of the Naval Research Laboratory in
Washington DC, the pressure cooker model was put to the test using observations
of a particular event known as the Bastille Day flare, which took place on 14
July 1998. In collaboration with researchers at the Smithsonian Astrophysical
Observatory in Cambridge, Massachusetts, the naval researchers used a land-based
magnetometer and NASA鈥檚 TRACE satellite to map out the magnetic fields in the
corona by watching infrared and UV radiation coming from the churning gases.
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The observations agree with the model鈥檚 assumption that the magnetic field of
the lid has a complex geometry involving four magnetic poles. Such a field
incorporates a 鈥渘ull point鈥�, where the field falls to zero. This provides a weak
spot that lets the accumulating energy below burst through the lid. 鈥淭he bottom
line is we have a much better idea of what is going on,鈥� says Aulanier.
But according to Ron Moore and his colleagues at the Marshall Space Flight
Center, Alabama, something more may be going on during the Sun鈥檚 most violent
and dramatic eruptions.
They propose a scenario in which a bundle of ionised gas held by twisted
magnetic fields gets caught in the downflow of one of the 300 000-kilometre-wide
convection loops that ferry material between the surface and the interior.
This magnetic bundle consumes material within the downflow, swelling until it
is fattened up 鈥渓ike a snake that swallowed a rat鈥�, says Moore. Eventually, the
bundle鈥檚 buoyancy breaks free of the current and it rises to the surface, where
it explodes as a flare.